Repulsing clays on drill bits

ABSTRACT

Bit balling is prevented by impressing a negative electrical charge on the drill bit by the use of bi-metallic electromotive potential differences, batteries or other electrical energy sources.

United States Patent 1191 McCaleb REPULSING CLAYS ON DRILL BITS StanleyB. McCaleb, Richardson. Tex.

Sun Oil Company, Dallas, Tex.

Oct. 10, 1972 Inventor:

Assignee:

Filed:

Appl. No.:

U.S. Cl 175/57, 166/65, 166/301, 175/320, 175/68 References Cited UNITEDSTATES PATENTS 5/1931 Baden 166/65 R X 3/1936 Lyons 204/180 R X 5/1942Hayward 175/64 X 1m.c1 Q. E21b7/00,E2lb 17/00- June 25, 1974 2,372,5753/1945 Hayward 166/301 2,466,239 4/1949 Holcombe 166/244 C 3,251,4275/1966 Ewing 175/320 3,294,184 12/1966 O'Brien 175/324 X 3,311,1813/1967 Fowler 175/410 3,734,181 5/1973 Shaffer 166/65 R X FOREIGNPATENTS OR APPLICATIONS 806,844 4/1951 Germany 175/57 154,201 3/1962U.S1S.R 166/301 [57] ABSTRACT Bit balling is prevented by impressing anegative electrica] charge on the drill bit by the use of bi-metallicelectromotive potential differences, batteries or other electricalenergy sources.

10 Claims, 4 Drawing Figures 1 REPULSING'CLAYS ON DRILL BITS BACKGROUNDOF THE DISCLOSURE This invention relates to the prevention of bitballing while drilling wells through fine formation materials such assticking clays.

A problem is encountered in drilling through earth formations whichcontain materials which stick to the drill bit or well pipe. Ifsufficient formation materials stick to the drill bit its cutting actioncan be greatly reduced and the drill string can become stuck in thehole. In the event the drill string becomes stuck, certain procedurescan be employed forfreeing the pipe. Initially, there is an attempt topull loose with the elevators. Next there are jars located in the drillstring that are utilized in an attempt to jar loose from the formation.

If the drill string cannot be extricated by the elevators or because ofthe lack or ineffectiveness of jars in the drill string then moreexpensive procedures must be employed. An attempt is usually made toremove all the pipe located above that stuck in the formation. The freepoint is determined and an explosive charge is utilized to loosen theconnection located directly above the free point. Once the connection isloosened, the pipe above the free point can be rotated to separate itfrom the pipe still stuck in the hole. Once the drill pipe above" thefree point has been removed, wash pipe is run into the wellbore which isof a larger diameter than the stuck drill pipe remaining in the hole.When the wash pipe has been loweredover the drill pipe water iscirculated down the wash pipe and up the annulus in an attempt to washaway the material binding the drill pipe. When it has been determinedthat the remaining portion of the drill string has been freed from theformation, a fishing tool is run into the wellbore in an attempt tospear the top point of drill pipe in order to retrieve the drill stringfrom the wellbore. If either the drill pipe cannot be freed by the washover process or it cannot be retrieved with fishing tools, whipstockingup hole must be utilized in order to bypass this area. A deflectingsurface is located above the stuck pipe and a new drill string is runinto the hole and deflected past the stuck pipe.

It can readily be seen that these procedures are expensive and greatcare should be taken to avoid having stuck drill pipe. The cost of theseprocedures is a great deal higher if sticking is encountered in offshorewells. Even if bit balling does not result in becoming stuck, thereduced drill rate plus the necessity to pull the drill string to cleanthe bit and collars prove to be quite costly.

Additionally, bit balling can cause blow outs if the drill string ispulled and a high pressure zone has been drilled. When the drill stringis pulled the balled up drill bit acts as a swab on the formation andcauses it to blow out. Because of the tremendous cost involved withtaming a blow out, great care is exercised to prevent excessive bitballing in such situations. Accordingly, more trips are made to cleanand check the bit when sticking formations are drilled. Thus, theeffective drilling rate is greatly reduced increasing substantially thecost of the well.

One formation material which proves most troublesome is shale composedof montmorillonite clay. This clay has two tetrahedral silica layers anda central octahedral alumina layenThis clay exhibits a large negativecharge. Exchangeable cations neutralize this negative charge and existsin a cation swarm at the surface of the clay. When these cations arefullyhydrated they are more loosely bonded to the clay due to itsdistance from the clay surface caused by the large radius of thehydrated molecule. The further these cations are from the clay, theweaker is the bond between the cations and the clay.

During a drilling operation a water base mud is generally usedeespecially in offshore areas where oil base muds are impractical due toanti-pollution rules. When the drill bit contacts these clays thecirculating water base mud acts to further hydrate the cations adjacentthe clay. As the drill bit contacts the clays, the hydrated cationspermit easier shearing apart of clay particles. Once these clays havebeen separated from the remaining clay matrix, they exist in a partiallydispersed hydrated state where they have their maximum plasticity.Because the clays exhibit a large negative charge they stick to themetallic drill string upon contact. In this state the clays will buildup on the drill string and will eventually disrupt the drillingoperation.

One method of combatting these sticking clays is to use an oil base mudhaving a water phase saturated with sodium and calcium chlorides. Thesechlorides control the clays by osmotic pressure, where water containedin the clay will transfer to the chloride saturated water phase of theoil base mud in attempting to equalize the salt concentrations.

In offshore areas, however, oil base muds are risky to use because ofanti-pollution rules. In these offshore areas, lime or gyp mud systemsare frequently used to inhibit the clay swelling. The lime or gypsystemprevents further hydration of the clays and often removes waterheld by the clays similar to the process described in the discussion ofoil base muds. These lime of gyp systems however, can be fairlyexpensive due to dilution of the mud requiring extensive addition of mudmaterial to keep up the mud weight. In those areas where oil base mudscan be used, the greater expense of such muds over normal drilling fluidmakes it desirous to find a less expensive drilling system for stickingclays.

Itis therefore an object of the present invention to provide an improveddrilling system for use in drilling through sticking earth formations.

SUMMARY OF THE INVENTION With this object in view, the present inventioncontemplates the prevention of adherence of earth formations to drillingapparatus by impressing a negative electrical charge on the drill bit.The preferred manner of impressing such negative charge is by locating,on a sub above the drill bit, a substantial amount of magnesium, therebyutilizing the differences in electromotive potential between steel andmagnesium. Downhole batteries, a current generator, or an electricalline run to an energy source at the surface can also be used.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of thelower part of a drill string having magnesium rings attached;

FIG. 1A is a block diagram of the cathodic action of the drill bit;

FIG. 2 is an elevational view of a drill string connected withelectrical energy, shown partly in cross section; r

.FIG. 3 illustrates an elevational view, shown partly in cross section,of the lower end of the drill string having a battery pack connectedwith the drill bit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 describes a wellbore 38having the lower end of a drill string shown therein. The drill stringportion shown has a drill bit sub 10' which is a short sub havinggenerally the same external dimensions as ordinary drill collars. Thissub has annular magnesium rings 18 located thereon. These magnesiumrings 18 can be placed on the sub 10 by machining down the mid portionof the sub so that'this portion of the sub has a lesser diameter thanthe sub ends. This machined portion of the sub is indicated at 42. Theannular magnesium rings 18 can then be heated to expand their internaldiameter to allow the magnesium ring 18 to be slipped over theunmachined portions of the drill bit sub 10 and onto the machined area42..Upon cooling the rings 18 becomes tightly attached to the machinedportion 42 of drill bit sub 10.

The drill bit sub 10 is positioned directly above the drill bit 12 whichbit is located at the bottom 36 ofwellbore 38. The drill bit is ofstandard design and has cones 32 with teeth 34, thereon. Drill bitsusually are made of a case hardened steel or alloy and drill bit teethusually are coated with a tungsten carbide material which has a highresistance to abrasion. Located at 32 between teeth 34 are magnesiuminserts to provide this material directly adjacent the drill bit teeth.Because magnesium is a fairly soft material it may be preferable tolocate the magnesium material in the interior of the bit or in areas onthe bit exterior which are not subject to extensive wear.

For the purposes of the following discussion, it is presumed thatformation 16 contains fine formation materials such as montmorilloniteclay or a large amount of other small size clay which have similaradhesive properties.

In drilling into a formation 16 having fine materials which causesticking and/r swelling the usual problem is the hydration of the clays.As the drill bit 12 engages the formation 16 the drill bit teeth 34 tendto shear apart portions of the hydrated clay. When these clays arepartially dispersed they exert their maximum plasticity and have a largenegative charge. Because of this large negative charge the clays areattracted to the drill bit and drill collars.

The magnesium located on the drill bit sub has a high electromotiveforce which impresses a negative charge on the drill bit 12. Magnesiumhaving the greater negative voltage acts as the anode and the drill bithaving the lesser negative voltage acts as the cathode. This is bestillustrated by reference to FIG. 1A where the magnesium ring is shown asthe anode at the left and the drill bit is shown as the cathode on theright. Drilling fluid is shown in between as an electro-,

lyte and the metallic pathway between the drill bit and magnesiumconnects the anode and cathode to complete the circuit.

As illustrated by FIG. 1 current flow follows the metallic pathway ofthe drill string from the magnesium to the drill bit when there isdrilling fluid present to act as an electrolyte. Because of thepotential difference between the magnesium anode and the drill bitcathode there is migration of electrons from the magnesium to v thedrill bit along the metallic drill string pathway. The

excess electrons received at the drill bit cathode will combine withpositively charged hydrogen ions in the electrolyte to form freehydrogen. With this loss of electrons, magnesium ions at the anode reactwith hydroxyl ions. The formation of hydrogen at the cathode drill bitalso aids in preventing clays from sticking by covering the surface ofthe drill bit. By this process the drill bit is protected from stickingclays with the magnesium acting as a sacrificial anode.

In the practical galvanic series commercially pure .magnesium exhibits l.75 volts while clean mild steel exhibits from -O.5 to O.8 volts. Shownbelow is a list of metals showing their voltage measured relative to asaturated copper-copper sulfate reference cell.

PRACTICAL GALVANIC SERIES (measured relative to a copper-copper sulfatereference electrode) METAL VOLTS Commercial pure magnesium Magnesiumalloy (6%, Al, 3% Zn, O.l5%

Aluminum alloy (5% Zn) l.()5 Comm. pure aluminum O.8 Mild steel (Cleanand shiny) 0.5 to O.8 Mild steel rusted 0.02 to 0.5 Cast iron 0.5Copper, brass, bronze 0.2

In the event the repelling force created by the magnesium drill bit subon the drill bit is not sufficient to repel the clay it may be necessaryto place the magnesium on the drill bit 12, itself. To prevent bitballing which is a term used for an excessive build up of clay on thedrill bit, it may be necessary to place magnesium inserts between thedrill bit teeth. This provides a multiplicity of sacrificial anodesimmediately adjacent the cutting teeth.- Another location forpositioning the magnesium could be between the rows of teeth on thedrill bit cone 32. Because the magnesium is not very resistant toabrasion, it may be necessary to locate the magnesium on the portion ofthe drill bit exterior which does not directly contact the formation 16.Since all the exterior of the bit may come into contact with theformation at one time or another, it may be preferable to position themagnesiumin the interior bore of the bit.

In addition to protecting the drill bit, magnesium rings can be utilizedto protect stabilizers, hole openers or any other area of the drillstring where sticking clays, etc., disrupt the drilling operation.

Referring next to FIG. 2 there is seen a standard drill string 11located in a wellbore 38 which has been drilled into a formation 16having fine materials including clays. At the end of the drill string 11is a standard drill bit 12 having teeth 14 thereon. These teeth 14 areshown engaging the bottom 36 of wellbore 38. Located immediately abovethe drill bit 12 is a drill bit sub 10. The sub is one of standarddesign and has been machined down to reduce the diameter of the midportion. An annular ring of insulating material 44 having retainingshoulders at the top and bottom is located around the machined diameterof the sub 10. Located between the retaining shoulders of the insulatingmaterial 44 is sacrificial anode material 46 such as high silicon iron.Located at the surface 40 is the top of the well 20 and a source ofelectrical energy 24 such as a generator or a connection with commercialelectric lines. An insulated electric line-22 extends between the sourceof electrical energy 24 and the sacrificial anode material 46. Thiselectrical line 22 is shown exiting the drill string 11 at 26 and isconnected with the generator 24. Generator 24 is grounded to the wellpipe by electrical connector 50. The electrical line 22 has been shownin the interior of the drill string, however, if mud flow down theinterior of the drill string proves troublesome it may be prefereble tolocate this electric line 22 on the exterior of the drill string.

With the apparatus described in FIG. 2 a negative electric charge can beimpressed on the drill bit 12 by supplying electric energy from source24 through line 22 which is connected with the sacrificial anode 46.This anode then acts in the same manner as the magnesium ring describedin FIG. 1. One advantage of this procedure is the ability to impress alarger negative charge on the drill bit than is possible with thebimetallic potential difference created by the magnesium ring 28described in FIG. 1. By appropriate switching, energy can be supplied toprotect the drill bit only at such times as sticking formations arebeing drilled. With the negative charge impressed on the drill bit, thenegatively charged clay is repelled and therefore does not stick to thedrill bit. In addition, the drill collars can also be negatively chargedto repel sticking clays by similar apparatus at appropriate locations.One disad vantage of this system is that it cannot be effectivelyutilized in deeper wells due to the loss in current caused by theinternal resistance of the electric line 22.

FIG. 3 describes a system which is effective in deeper formations thanthose which can effectively be protected by the apparatus described inFIG. 2. The lower portion of a drill string is located in a wellbore 38which penetrates formation 16 which includes sticking materials. Thedrill string comprises a drill bit sub below which is a drill bit 12which is shown engaging the bottom 36 of formation 16. The drill bit 12has drill bit cones 32 having teeth 14 thereon. Shown in the interior ofdrill bit sub 10 is battery pack 28 located along the wall of drill bitsub 10. The battery pack 28 includes a multiplicity of batteries whichare resistant to high temperatures and which are well insulated suchthat they will not be contaminated by drilling fluid or other liquids.An electrical connection 30 connects the battery pack 28 with asacrificial anode material 46, insulated by material 44 such as afiberized silica-epoxy material. The insulated anode is located on amachined down portion 42 of the sub 10.

With this apparatus it can easily be seen that a negative charge can beapplied to the drill bit 12 by energy supplied by the battery pack 28 tothe anode 46 to protect the drill bit 12 in the same manner as describedin FIGS. 1 and 2. Thus, when the teeth 14 on the drill bit 12 contactmaterials in the formation 16 which have a tendency to stick to thedrill bit, the charge provided by the battery pack 28 acts to repel thenegatively charged fine materials such as montmorillonate clay.

By impressing a negative charge by any of the methods shown in FIGS. 1-3a great savings can be had when drilling through troublesome formations.Sticking shales and like materials have caused substantial losses oftime and money including at times the complete loss of a wellbore. Thusa great savings can be had by preventing these materials from attachingto the drill string.

While particular embodiments of the present invention have been shownand described, it is apparent that changes and modifications may be madewithout departing from this invention in its broader aspects, andtherefore, the aim in the appended claims is to cover all such changesand modifications as fall within the true spirit and scope of thisinvention.

What is claimed is:

1. Process for drilling wellbores in earth formations containingmaterials which attach to the drill bit and drill collars comprising:positioning a metal with a substantially higher electromotive force thanthe drill bit at a point in the drill string directly adjacent the drillbit and between the drill collars and the drill bit for impressing anegative electrical charge on the drill bit during such time as theattaching materials are being drilled.

2. The process of claim 1 wherein the high electromotive force metallicmember is an annularly shaped magnesium metal which is press fit on aconventional.

drill bit sub.

3. A process for drilling wellbores through a sticking shale earthformation comprising: applying a rotating drill bit to the formation;and impressing a negative charge to the drill bit while drilling throughthe shale formation by locating directly adjacent the drill bit a metalhaving a higher electromotive force than that of the drill bit.

4. Apparatus for drilling through earth formations having a highconcentration of fine materials including clays comprising: a drill bit;drill pipe attached to the drill bit and extending to the surface; andmeans for impressing a negative charge on the drill bit, said impressingmeans being a metal which is located between the teeth on the drill bit.

5. The apparatus of claim 4 wherein the metal includes magnesium and islocated between the teeth of the drill bit.

6. A method for preventing the attachment of certain materials to adrill bit when drilling through earth formations having a highconcentration of materials which attach to the drill bit comprising:positioning a metal member with a higher electromotive force than thedrill bit in the drill string directly adjacent the drill bit and belowdrill collars in the drill string, rotating the drill string, andapplying the rotating drill bit to the formation.

7. The method of claim 6 and further including positioning the metalmember in the drill string by press fitting an annular metal sleeve onthe drill bit sub.

8. Apparatus for preventing the attachment of certain materials to adrill bit when drilling through earth formations having a highconcentration of materials which attach to the drill bit, including: adrill string extending to the surface and having drill collars and adrill bit at'its lower end; means positioned in the drill string belowthe drill collars for impressing a negative charge on the drill bit.

9. The apparatus of claim 8 wherein the impressing means is a metalmember having a higher electromotive force than the drill bit.

10. The apparatus of claim 8 wherein the impressing means is a magnesiumsleeve press fitted on the drill bit sub.

1. Process for drilling wellbores in earth formations containingmaterials which attach to the drill bit and drill collars comprising:positioning a metal with a substantially higher electromotive force thanthe drill bit at a point in the drill string directly adjacent the drillbit and between the drill collars and the drill bit for impressing anegative electrical charge on the drill bit during such time as theattaching materials are being drilled.
 2. The process of claim 1 whereinthe high electromotive force metallic member is an annularly shapedmagnesium metal which is press fit on a conventional drill bit sub.
 3. Aprocess for drilling wellbores through a sticking shale earth formationcomprising: applying a rotating drill bit to the formation; andimpressing a negative charge to the drill bit while drilling through theshale formation by locating directly adjacent the drill bit a metalhaving a higher electromotive force than that of the drill bit. 4.Apparatus for drilling through earth formations having a highconcentration of fine materials including clays comprising: a drill bit;drill pipe attached to the drill bit and extending to the surface; andmeans for impressing a negative charge on the drill bit, said impressingmeans being a metal which is located between the teeth on the drill bit.5. The apparatus of claim 4 wherein the metal includes magnesium and islocated between the teeth of the drill bit.
 6. A method for preventingthe attachment of certain materials tO a drill bit when drilling throughearth formations having a high concentration of materials which attachto the drill bit comprising: positioning a metal member with a higherelectromotive force than the drill bit in the drill string directlyadjacent the drill bit and below drill collars in the drill string,rotating the drill string, and applying the rotating drill bit to theformation.
 7. The method of claim 6 and further including positioningthe metal member in the drill string by press fitting an annular metalsleeve on the drill bit sub.
 8. Apparatus for preventing the attachmentof certain materials to a drill bit when drilling through earthformations having a high concentration of materials which attach to thedrill bit, including: a drill string extending to the surface and havingdrill collars and a drill bit at its lower end; means positioned in thedrill string below the drill collars for impressing a negative charge onthe drill bit.
 9. The apparatus of claim 8 wherein the impressing meansis a metal member having a higher electromotive force than the drillbit.
 10. The apparatus of claim 8 wherein the impressing means is amagnesium sleeve press fitted on the drill bit sub.